Long-Term Influence of Laser-Processing Parameters on - MDPI

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Long-Term Influence of Laser-Processing Parameters on (Super)hydrophobicity Development and Stability of Stainless-Steel Surfaces Peter Gregorˇciˇc 1, * , Marjetka Conradi 2 , Luka Hribar 1 and Matej Hoˇcevar 2 1 2

*

Faculty of Mechanical Engineering, University of Ljubljana, Aškerˇceva 6, 1000 Ljubljana, Slovenia; [email protected] Institute of metals and technology, Lepi pot 11, 1000 Ljubljana, Slovenia; [email protected] (M.C.); [email protected] (M.H.) Correspondence: [email protected]; Tel.: +386-1-477-1-172

Received: 24 October 2018; Accepted: 9 November 2018; Published: 11 November 2018







Abstract: Controlling the surface wettability represents an important challenge in the field of surface functionalization. Here, the wettability of a stainless-steel surface is modified by 30-ns pulses of a Nd:YAG marking laser (λ = 1064 nm) with peak fluences within the range 3.3–25.1 J cm−2 . The short(40 days), intermediate- (100 days) and long-term (1 year) superhydrophilic-to-(super)hydrophobic transition of the laser-textured surfaces exposed to the atmospheric air is examined by evaluating its wettability in the context of the following parameters: (i) pulse fluence; (ii) scan line separation; (iii) focal position and (iv) wetting period due to contact angle measurements. The results show that using solely a short-term evaluation can lead to wrong conclusions and that the faster development of the hydrophobicity immediately after laser texturing usually leads to lower final contact angle and vice versa, the slower this transition is, the more superhydrophobic the surface is expected to become (possibly even with self-cleaning ability). Depending on laser fluence, the laser-textured surfaces can develop stable or unstable hydrophobicity. Stable hydrophobicity is achieved, if the threshold fluence of 12 J cm−2 is exceeded. We show that by nanosecond-laser texturing a lotus-leaf-like surface with a contact angle above 150◦ and roll-off angle below 5◦ can be achieved. Keywords: laser surface engineering; wetting; superhydrophobic surfaces; laser material processing; surface modification

1. Introduction Inspired by hierarchical surface structures developed by nature [1,2], intensive research efforts employing different methods including chemical etching [3], two-step etching process [4], chemical vapor deposition [5], incorporation of inhibiting agents [6] and laser texturing [7,8] have been invested in the production of similar functionalized surfaces in the laboratory environment in the last decade. Several studies have shown that the surface morphology and chemistry on micro- and nanoscale can be efficiently controlled also by femtosecond [7–10], picosecond [11] and nanosecond [12–16] laser pulses as well as by continuous wave (CW) lasers [17]. Such laser-induced micro-/nanostructuring leads to a significant improvement of the surface functionality and opens up completely new possibilities in the field of surface engineering [18] for a wide range of applications in photonics, tribology, wettability, heat transfer, and biomedicine [19–24]. Laser texturing of different materials, including glasses, semiconductors, polymers and metals enables the production of surfaces with superior wetting properties, which may be exhibited as extreme water repellency [19,25,26], self-healing [27], self-cleaning [28], anti-icing [29], reduced drag in laminar and turbulent flows [30], significantly enhanced heat transfer [31,32], improved corrosion Materials 2018, 11, 2240; doi:10.3390/ma11112240

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resistance [33,34] and biodegradability [24]. The majority of these studies were made by ultrashort, i.e., pico-/femtosecond lasers. However, in the last three years several authors [12–15] have shown that similar surface functionalities can be achieved on metal surfaces (without additional coatings) by using more compact and cost-effective nanosecond laser systems that will importantly enhance the widespread use of the developed technology in different fields of application. In addition to the cost reduction, the real industrial applications also require flexibility, reliability and repeatability of the process [35]. Immediately after laser texturing, the surfaces usually become superhydrophilic with a static contact angle of θ = 0◦ —they are in a saturated Wenzel regime [15]. However, this (super)hydrophilic state is not stable. Consequently, such a textured surface exposed to the air at ambient conditions can develop hydrophobicity with contact angle exceeding 150◦ and roll-off angle (RoA) below 5◦ [13,16,25,34]. Under suitable conditions, the final, hydrophobic state becomes stable. Nevertheless, it seems that this development of water repellency depends on a narrow window of different parameters that are still not well understood. Therefore, the main aim of our work is to study the short-term (first two months after texturing) and the long-term (one year or longer) influence of different processing parameters, including fluence, scan line separation, focal position, and wetting during contact angle measurements on the development and stability of superhydrophobic surfaces after nanosecond-laser texturing of low carbon stainless steel in air. The low carbon stainless steel was chosen, since it is widely used in many areas of mechanical engineering and construction industry due to its excellent physical and mechanical properties [36]. Although several studies have been devoted to the examination of how processing parameters influence wettability of metallic surfaces [12–15,25,33,34,37], all of them were performed only within a short-term period after texturing (2 months or less). Therefore, the important goal of this research is to investigate the wettability development within a much longer period, specifically 1 year after surface texturing. The presented results lead to new important insights into the temporal wettability changes of laser-textured metallic surfaces. 2. Materials and Methods Materials. The samples made of commercially available AISI 316L stainless steel had the following chemical composition (in wt. %): Cr 16.9, Ni 10.04, Mo 2.07, Mn 1.84, Si 0.57, Cu 0.41, P 0.036, C 0.019, S 0.0009, V 0.077, Nb 0.015, N 0.044, and Fe the rest. They were laser-textured in the as received state without any polishing. The Sa roughness of the as received samples equaled 0.175 ± 0.02 µm. Before processing, all samples were ultrasonically cleaned in distilled water for 12 min and rinsed with ethanol. Laser texturing. Surfaces of the AISI 316L samples with dimensions of 20 × 20 × 1 mm3 were textured by a marking nanosecond Nd:YAG pulsed laser (LPKF, Ljubljana, Slovenia, OK DP10) radiating pulses with a wavelength of 1064 nm and duration of 30 ns. The laser beam was guided across the surface by a scanning head (Scanlab, Puchheim, Germany, SCANgine 14) equipped with an F-Theta focusing lens (focal length of 160 mm) resulting in a focal beam waist radius of 29 µm. All experiments were done by a constant pulse frequency of 25 kHz and using a constant marking speed of 150 mm/s, resulting in 90% overlap of consecutive spots. Some samples were textured only in the x direction (0◦ ), while others were textured in two directions (0◦ /90◦ ), first in the x (0◦ ) and then in the y (90◦ ). Experiments with different fluences. To examine the influence of laser fluence on the wettability development, we prepared 64 different samples. Half of them, i.e., 32 (labeled S1–S32), were textured in both directions (0◦ /90◦ ), while the others (labeled S33–S64) were textured only in x (0◦ ) direction. The distance between adjacent laser scanning lines (i.e., the scan line separation, ∆x and ∆y) equaled 50 µm for both directions and all samples. In these experiments, the peak fluences were varied within the range 3.3–25.1 J cm−2 . The detailed processing parameters are listed in Table S1. All the experiments were performed twice to confirm the repeatability of the results. Experiments with different micro-morphologies. The influence of different texturing patterns on the wettability development were examined in 16 samples, labeled S65–S80. Here, we tested the following additional scan line separations: 10 µm, 100 µm, 200 µm. At each scan line separation, two

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0◦ /90◦ and two 0◦ samples were textured. Each of these two samples was processed with one of the following peak fluences: F1 = 12.1 J cm−2 and F2 = 25.1 J cm−2 . The detailed processing parameters are listed in Table S2. Experiments at different focal positions. To study the influence of focal position, we prepared two samples, labeled S81 and S82 with different laser parameters. In all cases we used the same fluence F = 12.1 J cm−2 and 0◦ /90◦ pattern with scan line separation of ∆x = ∆y = 50 µm. Here, sample S81 was processed in the focal position (∆z = 0), while sample S82 was moved 600 µm from this position towards the F-Theta lens (∆z = −600 µm). Experiments with each S81–S82 sample were repeated 9 times. The processing parameters are collected in Table S3. Surface characterization. The samples morphology was characterized by using a scanning electron microscope (SEM; JEOL JSM-6500F) and by non-contact 3D optical microscopy (Alicona G4 3D optical Infinite-Focus Measuring device). Wettability measurements. Evolution of wettability was analyzed by measuring the static contact angle θ using a goniometer of our own design. The goniometer consists of a CCD camera (Basler AG, Ahrensburg, Germany, scA1400-17fm, 1.4 Mpx) equipped with a microscopic objective and micrometer syringe enabling delivery of distilled water droplet with a volume of 5 µL. When the image of a water droplet applied to the tested surface was captured, we fitted (i) the line to the interface between the solid surface and the surrounding air; and (ii) the circle to the liquid-gas interface. Here, the control points were manually added in a way that the circle mainly corresponds to the liquid-gas interface in the vicinity of both contact points between all three phases: gas, liquid and solid. The contact angle was estimated as the angle between the fitted circle and the fitted line, as shown by Figure S1 for the symmetrical and asymmetrical droplet. The short-term wettability development was studied within the first eight weeks after laser texturing. The first measurement was performed immediately after processing and was then repeated within the first two weeks every second day. Within the following two weeks, the wettability was measured every fourth day, while during the last four weeks, it was measured every second week. The long-term wettability of all samples was also examined one year after the texturing. After each measurement, the samples were dried by using a hot air gun (at 150 ◦ C). For comparison, the wettability of the as-received (unprocessed and unpolished) sample was measured and equaled 95.0 ± 6.4◦ . The reason of slightly hydrophobic nature of the as-received metallic samples most probably lies in micro/nanoroughness that leads to the Cassie-Baxter regime (that can turn, opposite to the Wenzel regime, hydrophilic material into a hydrophobic state). However, the Young angle of the base material should be measured on ideal (highly polished) surface [38,39]. Therefore, we performed this measurement on polished samples with Sa = 25 ± 2 nm. This way, the measured Young angle equals θ Y = 81.6 ± 5.7◦ . Studying the wetting influence on wettability development. Additional four samples, labeled S83-S86 were prepared with the same parameters as S81 (e.g., see Table S3) to study the effect of wetting due to static contact angle measurements on the wettability development. The evolution of wettability on these samples was evaluated within 40 days after texturing. Here, wettability of sample S83 was measured immediately after the laser texturing and then every 2nd day. The wettability on sample S84 was firstly measured 4 days after the texturing and then every 4th day, the wettability on sample S85 was firstly measured 8 days after the texturing and then every 8th day, while in the case of sample S86 we firstly measured wettability 16 days after the texturing and then every 16th day. Again, the samples were dried by using a hot air gun (at 150 ◦ C) after each measurement. 3. Results 3.1. Surface Morphology after Laser Texturing Overlapping of laser spots leads to the formation of micro(µ)-channels, as clearly visible on SEM micrographs in Figure 1a. Here, µ-channels induced by 0◦ texturing with scan line separation of ∆x

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= 50 µm are shown. It is clearly visible that µ-channel diameter Dµ (marked by the white arrows) depends on pulse fluence. This dependence can be described by using a Gaussian spatial profile [11], depends on pulse fluence. This dependence can be described by using a Gaussian spatial profile [11], where the fluence F(z,r) as a function of focal position z and radius r is given by: where the fluence F(z,r) as a function of focal position z and radius r is given by:   w02w2 2 22 2 FF((z,zr, )r )=FF − 2r /w ( z ) (1) 0 2 0 exp exp 2r / w (.z ) . (1) w 0 (2z )

w ( z)





Figure1.1. (a) peak fluences F0 (shown on the top of Figure (a) SEM SEM micrographs micrographsofofμ-channels µ-channelsatatdifferent different peak fluences F0 (shown on the topeach of micrograph); scan line separation in all cases equaled Δx = 50 μm. (b) Experimental (points) and each micrograph); scan line separation in all cases equaled ∆x = 50 µm. (b) Experimental (points) theoretical (line;(line; see see Equation (2)) (2)) dependence of μ-channels diameter on on peak fluence F0. F(c) and theoretical Equation dependence of µ-channels diameter peak fluence 0. Magnification of the selected area in in (a)(a) showing thethe appearance of of μ-cavities. (c) Magnification of the selected area showing appearance µ-cavities.

In Equation Equation (1), F00 stands r =r 0=and z = z0,=w0, 0 is waist radius In standsfor forthe thepeak peakfluence fluenceatat 0 and wthe the2 beam 1/e2 beam waist 0 is1/e at the focal (at z = (at 0), zwhile stands for the for beam as a function of z. If of thez.surface radius at theposition focal position = 0), w(z) while w(z) stands theradius beam radius as a function If the 2 2 /w22 ). Laser ablation surface is placed in the focal position, Equation (1) reduces to F ( r ) = F exp (− 2r 0 is placed in the focal position, Equation (1) reduces to F (r )  F0 exp(2r / w0 0). Laser ablation occurs in the region where the fluence F(r) exceeds the threshold fluence Fth for ablation. Therefore, occurs in the region where the fluence F(r) exceeds the threshold fluence Fth for ablation. Therefore, the µ-channel diameter Dµ as a function of peak fluence F0 can be described as: the μ-channel diameter Dμ as a function of peak fluence F0 can be described as: s   F0 Dµ = w0 2 ln  F0 . (2) (2) Dμ  w0 2 ln Fth  .

 Fth 

It should also be noted that the peak fluence F0 is twice higher than the average fluence F (also It should also be noted that the peak fluence F0 is twice higher than the average fluence F (also called the pulse fluence) normalized to the beam waist radius: called the pulse fluence) normalized to the beam waist radius: Ep F0 = 2F = 2 E , (3) F0  2 F  πw 2 02 p 2 , (3)

 w0

where Ep stands for the pulse energy. where Ep stands formicrographs the pulse energy. From the SEM (see Figure 1a and Table S4) we measured the µ-channel diameters. From the SEM micrographs (see Figure 1a and Table S4) we measured the μ-channel diameters. In the case of the highest peak fluence F0 (the last micrograph in Figure 1a), the Dµ cannot be In the case of the highest peak fluence F0 (the last micrograph in Figure 1a), the Dμ cannot be determined, since it exceeds scan line separation, ∆x = 50 µm. determined, since it exceeds scan line separation, Δx = 50 μm. The measured diameters Dμ as a function of peak fluence F0 are shown as dots in graph in Figure 1b. Here, the solid line presents the fit of Equation (2) to the measured data by using least-square method. From this fit we obtained the fluence threshold as Fth = 3.8 J cm−2. The fit further enabled us to estimate the μ-channel diameter at F0 = 25.1 J cm−2; it equals Dμ = 56 μm > ∆x and, consequently,

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The measured diameters Dµ as a function of peak fluence F0 are shown as dots in graph in Figure 1b. Here, the solid line presents the fit of Equation (2) to the measured data by using least-square method. From this fit we obtained the fluence threshold as Fth = 3.8 J cm−2 . The fit further enabled us to estimate the µ-channel diameter at F0 = 25.1 J cm−2 ; it equals Dµ = 56 µm > ∆x and, consequently, results in overlapping of the adjacent µ-channels. This calculated result is experimentally confirmed by µ-channel diameter measurements at the same fluence but for ∆x = 100 µm (see Table S4). Since the surface topography significantly depends on the combination of peak fluence and scan line separation, the variation of these two parameters can be used for structuring different surface topographies. In a special case, when Dµ is just slightly smaller than scan line separation ∆x, the µ-cavities appear on the border between two adjacent µ-channels as clearly visible from Figure 1c that is the magnification of the selected area (marked by the white rectangle) in the third micrograph of Figure 1a. Such µ-cavities play an important role in engineering of surfaces for enhanced heat transfer, as explained, demonstrated and proved in Refs. [31,32]. The importance of the scan line separation is shown in Figure 2. Here, 0◦ /90◦ texturing with different scan line separations ∆x = ∆y was used at two different fluences. The surface was first textured in x (0◦ ) and then in y (90◦ ) direction. As it has already been shown [34], the second beam pass is more pronounced. From Figure 2 it can be seen that decreasing scan line separation and/or increasing (peak) fluence leads to higher surface porosity. This is caused by to the increased overlap between the adjacent µ-channels. The high-magnification SEM images are presented in Figures S2–S4. For surfaces in Figure 2, we also measured the average surface roughness Sa by using 3D microscopy. The results, listed in Table 1, indicate that the lowest value of Sa is measured for ∆x = ∆y = 10 µm for both fluences due to the highest surface porosity of the two surfaces. A further increase in Sa is observed with increased scan line separation until Dµ is smaller than ∆x = ∆y, reaching a peak value for both fluences at ∆x = ∆y = 50 µm. After this point, the additional increase of scan line separation results in decreased average surface roughness due to the appearance of an unprocessed area between two adjacent, well-separated µ-channels. Similar dependence on surface roughness as a function of scanning line separation was shown by Conradi et al. [20]. Table 1. Average surface roughness (Sa ) for surfaces from Figure 2. ∆x, ∆y (µm)

Sa (µm) F0 = 12.1 J

10 25 50 100

0.85 1.13 4.54 1.71

cm−2

F0 = 25.1 J cm−2 1.47 1.70 5.40 4.02

Typical 3D surface topographies are shown in Figure 3. When 0◦ texturing is applied, the lines appear on surfaces as visible in Figure 3a. On the other hand, the comparison between 0◦ /90◦ texturing in the focal and out of the focal position (at z = −600 µm, i.e., 600 µm towards the focal lens) is revealed in Figure 3b,c, respectively. It can be seen that in this case, the µ-channels are opened in the last scanning direction (90◦ ) as has been already been shown by Trdan et al. [34]. Here, the µ-holes appear where both texturing directions cross each other.

different scan line separations ∆x = ∆y was used at two different fluences. The surface was first textured in x (0°) and then in y (90°) direction. As it has already been shown [34], the second beam pass is more pronounced. From Figure 2 it can be seen that decreasing scan line separation and/or increasing (peak) fluence leads to higher surface porosity. This is caused by to the increased overlap Materials 2240 15 between2018, the11, adjacent μ-channels. The high-magnification SEM images are presented in Figures6 of S2– S4.

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For surfaces in Figure 2, we also measured the average surface roughness Sa by using 3D microscopy. The results, listed in Table 1, indicate that the lowest value of Sa is measured for ∆x = ∆y = 10 μm for both fluences due to the highest surface porosity of the two surfaces. A further increase in Sa is observed with increased scan line separation until Dμ is smaller than ∆x = ∆y, reaching a peak value for both fluences at ∆x = ∆y = 50 μm. After this point, the additional increase of scan line separation results in decreased average surface roughness due to the appearance of an unprocessed area between two adjacent, well-separated μ-channels. Similar dependence on surface roughness as a function of scanning line separation was shown by Conradi et al. [20]. Table 1. Average surface roughness (Sa) for surfaces from Figure 2.

Δx, Δy (μm) 10 25 50 100

Sa (μm) F0 = 12.1 J cm−2 F0 = 25.1 J cm−2 0.85 1.47 1.13 1.70 4.54 5.40 1.71 4.02

Typical 3D surface topographies are shown in Figure 3. When 0° texturing is applied, the lines appear on surfaces as visible in Figure 3a. On the other hand, the comparison between 0°/90° texturing in the focal and out of the focal position (at z = −600 μm, i.e., 600 μm towards the focal lens) is revealed inSEM Figures 3b,c, respectively. It can bebyseen that in this case,F0the areoftop opened in Figure 2. 2. SEM micrographs surfaces processed by different fluences F0μ-channels (shown ontop the of Figure micrographs ofofsurfaces processed twotwo different fluences (shown on the each eachscanning column) and with different line separations = Δy (shown on the left-hand side of eachthe μthe last direction (90°) asscan has been already been shown Trdan et of al.each [34]. Here, column) and with different scan line separations ∆x = ∆y Δx (shown on the by left-hand side row). holesrow). appear where both texturing directions cross each other.

◦ textured Figure3.3. 3D 3Dprofile profileofof(a) (a)00° texturedsurface surfaceatatfocal focalposition, position,(b) (b)0◦0°/90° textured surface surface at atfocal focal Figure /90◦ textured ◦ ◦ position and (c) 0°/90° textured surface out of focal position at z = −600 μm (towards the laser). All position and (c) 0 /90 textured surface out of focal position at z = −600 µm (towards the laser). −2 with − 2 surfaces are processed at F 0 = 12.1 J cm Δx = Δy = 50 μm. All surfaces are processed at F0 = 12.1 J cm with ∆x = ∆y = 50 µm.

From From the the 3D 3Dprofiles profileswe wemeasured measured the the peak-to-valley peak-to-valley amplitudes amplitudes (PVA) (PVA) as as aa function function of of peak peak fluence fluence FF00,, when when surface surface is is textured textured with with scan scan line lineseparation separation ∆x Δx==∆y Δy==5050µm. μm. The The obtained obtained dependence is presented in Figure 4. Here, the PVA value was obtained as average peak-to-valley amplitude across the black dashed lines in Figure 3. The black dots in Figure 4 represent average PVA for 0°/90° texturing in focal position, while the gray dots stand for average PVA for 0° texturing in the focal position. The average PVA at F0 = 12.1 J cm−2 when the sample is placed 600 μm towards the focusing lens is presented for the reference in Figure 4 as the orange square. The error bars in Figure

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dependence is presented in Figure 4. Here, the PVA value was obtained as average peak-to-valley amplitude across the black dashed lines in Figure 3. The black dots in Figure 4 represent average PVA for 0◦ /90◦ texturing in focal position, while the gray dots stand for average PVA for 0◦ texturing in the focal position. The average PVA at F0 = 12.1 J cm−2 when the sample is placed 600 µm towards the focusing lens is presented for the reference in Figure 4 as the orange square. The error bars in Figure 4 show standard Materials 2018, 11, xdeviation. FOR PEER REVIEW 7 of 15

Figure 4. 4. The The average average peak-to-valley peak-to-valley amplitude amplitude as as aa function function of of the the peak peak fluence fluence FF00.. The The dependence dependence Figure ◦ ◦ ◦ for 00°/90° dots) and and for /90 texturing texturingin inaa focal focal position position (black (black dots), dots), 0° 0 texturing texturing in in aa focal focal position position (gray (gray dots) −2 (the ◦ /90◦texturing −2 (the μm at at F0 F=0 12.1 J cm orange square) are 00°/90° texturingout outofofthe thefocal focalposition positionatatz z= =−600 −600 µm = 12.1 J cm orange square) shown. are shown.

It is clearly clearly visible visible that PVA PVA increases increases approximately approximately linearly linearly for for peak peak fluences fluences FF00 between between the ◦ threshold value value FFththand andthe thesaturated saturated value In case the case 0 texturing, the increases PVA increases for value FsatF. sat In.the of 0°of texturing, the PVA for factor −2 ) and the saturated value PVA −2) and factor k0◦μm/(J = 0.81cm µm/(J cm 16 µm is Fsat−2.=On 22.6 of k0° =of 0.81 the saturated value PVAsat ≈ 16 μmsat is ≈ achieved atachieved Fsat = 22.6at J cm theJ −2 . On the other hand, the linear coefficient k ◦ ◦ /90◦ −2)1.92 ◦ = cm µm/(J cm−2for ) is 0°/90° twice higher for 0In other hand, the linear coefficient k0°/90° = 1.92 μm/(J cm is twice higher texturing. this 0 /90 − 2 −2 texturing. In this case, thePVA saturated = 15.6 cm . In case case, the saturated value sat ≈ 23value μm isPVA obtained atµm Fsatis= obtained 15.6 J cmat. FIn of Jprocessing out of sat ≈ 23 satcase ◦ texturing in a focal processing outPVA of thevalue focus, PVA value is between the values 0◦ and 0in◦ /90 the focus, the is the between the values for 0°and 0°/90°for texturing a focal position (for the position (forfluence). the same pulse fluence). same pulse 3.2. 3.2. Influence Influence of of Pulse Pulse Fluence Fluence on on Surface Surface Wetting Wetting Properties Properties The thethe static contact angle. Although some authors [40] The wettability wettabilitywas wasevaluated evaluatedbybymeasuring measuring static contact angle. Although some authors denounce the static contact angle analysis since thisthis angle cancan reach anyany value within thethe range of [40] denounce the static contact angle analysis since angle reach value within range angles between the advancing and the receding contact angle, we made such an analysis to compare of angles between the advancing and the receding contact angle, we made such an analysis to our results with previous [12–15,25,33,34,37] that used similar methodology. However, compare our results with experiments previous experiments [12–15,25,33,34,37] that used similar methodology. we also listwe thealso roll-off angle (RoA)angle where applicable. However, list the roll-off (RoA) where applicable. The short-term development of the static water contact angle we examined on surfaces that The short-term development of the static water contact angle we examined on surfaces that were −2 were processed with a scan line separation of ∆x = ∆y = 50 µm at two different fluences, 12.1 J cm −2 processed with a scan line separation of Δx = Δy = 50 μm at two different fluences, 12.1 J cm and 5.5 and −2 (Figure J cm−2 (Figure As evident temporal contact angledevelopment development(Figure (Figure 6a), 6a), the J cm5.5 5). As 5). evident fromfrom the the temporal contact angle the −−22 is developed gradually, with linear (super)hydrophobicity on the surface processed at 12.1 J cm (super)hydrophobicity on the surface processed at 12.1 J cm is developed gradually, with linear increase of the the static staticwater watercontact contactangle angle(κ(κ==θ θmax /tmax 11per degday) per in day) the13first 13until daysstable until max increase of /tmax ≈ 11≈ deg thein first days stable water-repellency was achieved. After this time, the surface then remains superhydrophobic water-repellency was achieved. After this time, the surface then remains superhydrophobic with a ◦ ± 0.50. We have measured with stable contact = ±θ f2° ≈and 159a◦ RoA ± 2◦of and a ±RoA 4.3have stableacontact angle ofangle θmax =of θf θ≈max 159° 4.3° 0.50.ofWe measured the wettability the wettability of the same sample again after year (i.e., so called long-term measurement) and of the same sample again after one year (i.e., so one called long-term measurement) and the contact angle ◦ ± 3◦ , while the RoA was still below 5◦ . This proves the long-term the contact angle equaled to 157 equaled to 157° ± 3°, while the RoA was still below 5°. This proves the long-term stability of these stability samples.of these samples.

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Figure 5. Short-term development of (a) hydrophobicity with a stable contact angle (at F0 = 12.1 J cm−2) and (b) hydrophobicity with an unstable contact angle (at F0 = 5.5 J cm−2).

On the contrary, the surface processed with a lower peak fluence, 5.5 J cm−2, does not develop a stable contact angle in the short-term (2 month) period (Figure 6b). At the beginning, its behavior is similar to the surface textured at a higher fluence, but the contact angle increases more rapidly (κ = θmax/tmax ≈ 20 deg per day). After the first 5 days, the surface saturates in the hydrophobic limit by achieving the contact angle θmax = 100°, with no RoA since the water droplet remains stuck to the surface even when inclined for 90°. The surface remains (stably) hydrophobic for approximately 30 days; after this time, the contact angle starts to decrease and after two months it reaches the “final” contact angle of θf = 65° (with no RoA). However, the long-term measurement after one year revealed thatFigure stable equals 91°development (with no RoA), which is close towith the contact angle of (at the non-processed Figure5.θ 5.f Short-term Short-term of hydrophobicity stable contact angle (at F 0 =F12.1 J cm−2 development of(a) (a) hydrophobicity withastatic a stable contact angle J) 0 = 12.1 −2). − 2 − 2 and (b) hydrophobicity with an unstable contact angle (at F 0 = 5.5 J cm sample. cm ) and (b) hydrophobicity with an unstable contact angle (at F0 = 5.5 J cm ). On the contrary, the surface processed with a lower peak fluence, 5.5 J cm−2, does not develop a stable contact angle in the short-term (2 month) period (Figure 6b). At the beginning, its behavior is similar to the surface textured at a higher fluence, but the contact angle increases more rapidly (κ = θmax/tmax ≈ 20 deg per day). After the first 5 days, the surface saturates in the hydrophobic limit by achieving the contact angle θmax = 100°, with no RoA since the water droplet remains stuck to the surface even when inclined for 90°. The surface remains (stably) hydrophobic for approximately 30 days; after this time, the contact angle starts to decrease and after two months it reaches the “final” contact angle of θf = 65° (with no RoA). However, the long-term measurement after one year revealed that stable θf equals 91° (with no RoA), which is close to the static contact angle of the non-processed sample.

Figure6.6.Short-term Short-termcontact contactangle angledevelopment developmentfor for(a) (a)surface surfaceexhibiting exhibitinghydrophobicity hydrophobicitywith withstable stable Figure −2 and (b) surface exhibiting hydrophobicity with unstable − 2 contact angle, processed at F 0 = 12.1 J cm contact angle, processed at F0 = 12.1 J cm and (b) surface exhibiting hydrophobicity with unstable contactangle, angle,processed processedatatF0F0==5.5 5.5J Jcm cm−−22.. Both contact Both surfaces surfaces were were processed processed with with Δx ∆x == Δy ∆y ==50 50 μm. µm. −2 , doeswith Tothe evaluate and the wettability after laser texturing different On contrary, thecompare surface processed with adevelopment lower peak fluence, 5.5 J cm not develop weangle propose to use the following characteristics as wettability metrics (see also 6b): aparameters, stable contact in the short-term (2 month) period (Figure 6b). At the beginning, itsFigure behavior is similar to the surface textured at a higher fluence, but the contact angle increases more rapidly • the maximal contact angle, θmax which is defined as a static (apparent) contact angle achieved (κ = θ max /tmax ≈ 20 deg per day). After the first 5 days, the surface saturates in the hydrophobic limit within the measured (short- or long-term) period; by achieving the contact angle θ max = 100◦ , with no RoA since the water droplet remains stuck to • the time, tmax, defined as the time in which the maximal contact angle, θmax, is achieved; the surface even when inclined for 90◦ . The surface remains (stably) hydrophobic for approximately • and the final contact angle θf, defined as the apparent contact angle, measured at the end of the 30 days; after this time, the contact angle starts to decrease and after two months it reaches the evaluating period—as clearly demonstrated by Figure 6b, this is a very vague parameter/metric ◦ (withdevelopment “final”Figure contact angle of θ contact no RoA). However, the long-term measurement after one year 6. Short-term for (a) surface exhibiting hydrophobicity with stable f = 65 angle ◦ −2 contact processed at F 0 = 12.1 J cm and (b)which surfaceisexhibiting withangle unstable revealed thatangle, stable θ f equals 91 (with no RoA), close to hydrophobicity the static contact of the contact angle, processed at F0 = 5.5 J cm−2. Both surfaces were processed with Δx = Δy = 50 μm. non-processed sample. To evaluate and compare the wettability development after laser texturing with different To evaluate and compare wettability development after laser texturing with different parameters, we propose to use the the following characteristics as wettability metrics (see also Figure 6b): parameters, we propose to use the following characteristics as wettability metrics (see also Figure 6b): • the maximal contact angle, θ max which is defined as a static (apparent) contact angle achieved • within the maximal contact(shortangle,or θmax which isperiod; defined as a static (apparent) contact angle achieved the measured long-term) within the measured (short- or long-term) period; • the time, tmax , defined as the time in which the maximal contact angle, θ max , is achieved; • the time, tmax, defined as the time in which the maximal contact angle, θmax, is achieved; • and the final contact angle θf, defined as the apparent contact angle, measured at the end of the evaluating period—as clearly demonstrated by Figure 6b, this is a very vague parameter/metric

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and the final contact angle θ , defined as the apparent contact angle, measured at the end of the 9 of 15 evaluating period—as clearly demonstrated by Figure 6b, this is a very vague parameter/metric especially that are are presented presented and and discussed discussedby bythe themajority majorityofofthe the especiallyin inshort-term short-term measurements measurements that published papers [12–15,25,33,34,37] reporting on the laser-induced wettability control. published papers [12–15,25,33,34,37] reporting on the laser-induced wettability control.

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The maximal and the “final” contact angles of surfaces, processed with different laser fluences (for The maximal and the “final” contact angles of surfaces, processed with different laser fluences ∆x = ∆y = 50 µm) were evaluated as a function of pulse fluence. The results are presented in Figure 7. (for Δx = Δy = 50 μm) were evaluated as a function of pulse fluence. The results are presented in Here, the results of unstable wettability (similar behavior as in Figure 6b) are marked by the orange Figure 7. Here, the results of unstable wettability (similar behavior as in Figure 6b) are marked by the circles, while the stable superhydrophobic surfaces (e.g., with behavior similar to that in Figure 6a) orange circles, while the stable superhydrophobic surfaces (e.g., with behavior similar to that in are shown with the gray circles. Figure 7a shows the maximal contact angle as a function of fluence Figure 6a) are shown with the gray circles. Figure 7a shows the maximal contact angle as a function and indicates the threshold peak fluence of 12 J cm−2 for achieving stable wettability. This threshold of fluence and indicates the threshold peak fluence of 12 J cm −2 for achieving stable wettability. This fluence was determined as an average value of fluences used to process the last unstable sample and threshold fluence was determined as an average value of fluences used to process the last unstable the first stable sample. In this case, the threshold final contact angle of 140◦ is exceeded, as visible from sample and the first stable sample. In this case, the threshold final contact angle of 140° is exceeded, Figure 7b showing the final contact angle as a function of the peak fluence. as visible from Figure 7b showing the final contact angle as a function of the peak fluence.

Figure7.7.(a) (a)Maximal Maximal contact contact angles angles and (b) final contact contact angles Figure angles as as aa function functionof ofthe thepeak peakfluence fluenceFF0.0 . Allresults resultsare aremeasured measuredfor for∆x Δx== ∆y Δy == 50 µm. μm. All

3.3. 3.3.Influence InfluenceofofScan ScanLine Line Separation Separation on on Surface Surface Wetting Wetting Properties Properties We wettability development developmentfor fordifferent different Wehave havealso alsoanalyzed analyzedthe the short-term short-term and and the the long-term long-term wettability ◦ texturing), ∆x = 10 µm, 100 µm, 200 µm and for the net-textured surfaces scan line separations (0 scan line separations (0° texturing), Δx = 10 μm, 100 μm, 200 μm and for the net-textured surfaces ◦ /90◦ ) ∆x = ∆y = 10 µm, 100 µm, 200 µm. All the samples were processed at the same fluence (0(0°/90°) Δx = Δy = 10 μm, 100 μm, 200 μm. All the samples were processed at the same fluence of 25.1 −2 ofJ 25.1 J cm−2 (significantly exceeding the threshold fluence development the stable stable cm (significantly exceeding the threshold fluence for for the the development of ofthe superhydrophobicity—see superhydrophobicity—seeFigure Figure7a). 7a). Short-term include a 40-day contact angle evaluation; the wettability was measured Short-termmeasurements measurements include a 40-day contact angle evaluation; the wettability was again after 100 days (intermediate-term measurement), while the long-term measurements of the measured again after 100 days (intermediate-term measurement), while the long-term measurements contact angle were after 1 after year. 1The results are presented in Figure 8. of the contact angleperformed were performed year. The results are presented in Figure 8. The with ∆x ∆x= =100 100 = 10turned µm turned highly hydrophobic with The samples samples with μmµm andand Δx =∆xΔy==∆y 10 μm highly hydrophobic with contact ◦ . The angles of around The 141 samples withsamples scan-linewith separations of ∆x = 200 μm and Δx== Δy 200 μm contact angles of 141°. around scan-line separations of ∆x 200= µm and ◦ remained moderately hydrophobic withhydrophobic contact angles and 132°. However, the◦ . ∆x = ∆y = 200 µm remained moderately withbetween contact 125° angles between 125 and 132 superhydrophobicity as defined by as Wang and by Jiang [41]and was—in only for the However, the superhydrophobicity defined Wang Jiang this [41] case—achieved was—in this case—achieved surfaces with smaller scan-line separations, (e.g., Δx = Δy = 50 μm—Figure 7; and Δx = Δy = 10∆x μm— only for the surfaces with smaller scan-line separations, (e.g., ∆x = ∆y = 50 µm—Figure 7; and = ∆y ◦ ◦ 8a). Here,8a). theHere, measured contact angles upwere to 159° while), the measured RoA =Figure 10 µm—Figure the measured contactwere angles up to(>150°), 159 (>150 while the measured ◦ ± 0.5 ◦ . That equaled 3.0°3.0 ± 0.5°. That means that ifthat thisifsurface is tilted for more than than 3°, the droplet roll off RoA equaled means this surface is tilted for more 3◦ ,water the water droplet roll the surface and cleans the dust pieces put on the surfaces—it expresses the self-cleaning effect [42] off the surface and cleans the dust pieces put on the surfaces—it expresses the self-cleaning effect [42] (seealso alsoFigure FigureS5). S5). (see

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Figure 8. Short-term, intermediate-term and long-term static water contact angle development on surfaces with the following scan line separations: (a) ∆x = ∆y = 10 µm; (b) 100 µm; and (c) and 200 µm. Figure 8. Short-term, intermediate-term and long-term static water contact angle development on surfaces with 3.4. Influence scan of Focal on Surface Properties the following line Position separations: (a) Δx =Wetting Δy = 10 μm; (b) 100 μm; and (c) and 200 μm.

Some recent results [14] show that wettability gradients can be achieved by processing a metallic 3.4. Influence of Focalfocal Position on Surface Wetting we Properties sample at different positions. Therefore, have examined the short-term and the long-term wettability of the laser-textured surfaces as a function of the focal position. Thebyresults for thea surfaces Some recent results [14] show that wettability gradients can be achieved processing metallic processed the focal position (i.e., at z = 0) andwe at zhave = −600 µm (that our case equals to 22% of the sample at in different focal positions. Therefore, examined theinshort-term and the long-term Rayleigh length, = 2.7 mm) are presented in Figure 9. wettability of thezlaser-textured surfaces as a function of the focal position. The results for the surfaces R Figurein9a shows development θ maxat/tzmax , i.e.,μm the(that average increase in a contact angle processed the focal the position (i.e., at z =rate 0) and = −600 in our case equals to 22% of the ◦ per day from thezsuperhydrophilic state (the Rayleigh length, R = 2.7 mm) are presented insaturated Figure 9. Wenzel regime, with θ = 0 ) to the maximal contact angle9aθ max , measured within 2 months (the so-called short-term period). in This indicates that per the Figure shows the development rate θmax /tmax , i.e., the average increase a contact angle surface processed out of the focus box in Figure expresses hydrophilic behavior day from the superhydrophilic state(the (theleft saturated Wenzel9a) regime, with aθ more = 0°) to the maximal contact and hydrophobicity slower than the sample processed in the focal position. that However, such angledevelops θmax, measured within 2 months (the so-called short-term period). This indicates the surface wettability differences are not stable by time, since the superhydrophobic state was achieved by processed out of the focus (the left box in Figure 9a) expresses a more hydrophilic behavior both and surfaces a short-term periodthan (2 months after the processing) as focal can beposition. easily seen from thesuch box developswithin hydrophobicity slower the sample processed in the However, plots in Figure 9b presenting maximal contact of the surfaces, processed in and out the wettability differences are notthe stable by time, since angles the superhydrophobic state was achieved byofboth focal position. The wettability of the same surfaces was evaluated again after one year and the results surfaces within a short-term period (2 months after the processing) as can be easily seen from the box are presented Figure 9c. the maximal contact angles of the surfaces, processed in and out of the plots in Figurein9b presenting focal position. The wettability of the same surfaces was evaluated again after one year and the results are presented in Figure 9c.

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Figure9.9.The Thewettability wettabilityparameters parametersasasa afunction functionofofthe thefocal focalposition positionz.z.(a) (a)The Theaverage averagedevelopment development The wettability parameters function the focal position (a) The average development Figure rate,measured measuredwithin within222months; months;(b) (b)the themaximal maximalcontact contactangle anglewithin within2 2months; months;and and(c) (c)the thefinal final measured within months; (b) maximal contact angle within months; and rate, −−22 and Δx −2 contactangle angleafter after11year. year.The Thesample samplewere wereprocessed processedbybyF0F 0 = 12.1 J cm = Δy = 50 μm. contact = 12.1 J cm ∆x = ∆y = 50 µm. 0 = 12.1 J cm and Δx = Δy = 50 μm.

3.5. The Effect ofofWetting Period on Hydrophobicity 3.5.The TheEffect Effectof WettingPeriod Periodon onHydrophobicity HydrophobicityDevelopment Development 3.5. Wetting Development It is impossible to measure the surface wettability without wetting surface. Therefore, impossibletotomeasure measurethe thesurface surfacewettability wettabilitywithout without wetting thethe surface. Therefore, four ItItisisimpossible wetting the surface. Therefore, four four different surfaces (S83–S86; Table S3) were textured with the same processing parameters differentsurfaces surfaces(S83–S86; (S83–S86;Table TableS3) S3)were weretextured texturedwith withthe thesame sameprocessing processingparameters parametersΔx Δx==Δy Δy== different −2 ), but measurements on each of them were conducted at different ∆x =μm; ∆yFF = 5012.1 µm;J Jcm Fcm =−2),12.1 cm 0 −2 50μm; ),but butJmeasurements measurements oneach eachofofthem themwere wereconducted conductedatatdifferent differentperiods. periods.The The 50 0 0==12.1 on periods. The first sample (S83) underwent measurements every second day, the S84 every fourth day, firstsample sample(S83) (S83)underwent underwentmeasurements measurementsevery everysecond secondday, day,the theS84 S84every everyfourth fourthday, day,the theS85 S85 first the S85eighth everyday, eighth day,measurements while measurements the S86 sample were conducted sixteenth day. every eighth day,while while measurements onthe theon S86 sample wereconducted conducted everyevery sixteenth day.The The every on S86 sample were every sixteenth day. The wettability development for allmeasured the measured surfaces is shown in Figure wettability development forall all the measured surfaces shown Figure 10. 10. wettability development for the surfaces isisshown ininFigure 10.

Figure10. 10. Contact angle development for different different wetting periods due contact angle angle Figure 10. Contact angle development for wetting periods due totomeasurements. contact Contact angle development for different wetting periods due to contact angle measurements. measurements.

The presented results indicate that the surface wetting also influences the wettability transition, The presented results indicate thatthe thesurface surface wetting alsoinfluences influences theawettability wettability transition, but itThe haspresented no significant influence onthat the final results—all thealso surfaces achieved similar contact angle results indicate wetting the transition, ◦no ◦influence ithas has significant onthe thefinal finalresults—all results—all thesurfaces surfaces achieved similar contactangle angle ofbut θ it≈ 150no and RoA < 5influence after approximately 2 months. the The wettability of all aofasimilar these surfaces were but significant on achieved contact ofθθ≈≈150° 150° andRoA RoA 150◦ and RoA < 5◦ ) is achieved even for the smallest scan line separation (i.e., ∆x = ∆y = 10 µm; Figure 8a). The presented long-term measurements, in contrast to the existing literature [13], suggest that the highest final contact angle is achieved for highest values of the totally absorbed energy. In case of smaller scanning line separation, more pulses (i.e., more total energy) are needed to process the whole surface. Therefore, from the presented results, it can be concluded that a higher amount of the totally absorbed energy leads to a slower development of superhydrophobicity after laser texturing. Furthermore, the results in Figures 6 and 8 indicate that this slower wettability transition leads to higher final contact angles (more hydrophobic surfaces). The importance of the long-term wettability measurements is additionally proved by the examination of how the focal position influences on surface wettability (Figure 9). Here, the short-term measurements lead to the (wrong) conclusion that the processing out of the focus causes a more hydrophilic behavior. A similar conclusion was developed by Ta et al. [14] who demonstrated that wettability gradients can be achieved by processing the metallic sample at different focal positions; unfortunately, their measurements were limited to less than 2 months. Our long-term contact angle measurements reveal that such wettability gradients are not stable by time. Instead, the presented results clearly indicate that the focal position (within the range that still enable pulse fluences that are high enough for the laser ablation) mainly influences the wettability transition, but not the final contact angle. Additionally, these results prove that the slower the hydrophilic-to-hydrophobic transition is, the higher the final contact angle can be expected. Not only the processing parameters, but also the measurements of the contact angle itself influence the wettability transition. This happens because the measurement always interferes with the measured result—in this case, it is impossible to measure the contact angle without wetting the surface by putting a droplet on it. The influence of the contact angle measurements on wettability behavior (Figure 10) indicates that frequent measurements speed up the wettability transition, but have no significant influence on the final wettability. Therefore, the same period of the contact angle measurements should be used, when one aims to compare the wettability transition of different surfaces. On the contrary, it seems that the measurement frequency is not very important when only the final contact angles (measured after steady-state conditions are achieved) are investigated and/or compared.

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5. Conclusions The as-received stainless-steel surfaces have been textured with nanosecond pulses using different pulse fluences and different scan line separations. The short- (within 40 days), intermediate- (within 100 days) and long-term (after one year) superhydrophilic-to-(super)hydrophobic transition was examined in the context of the following processing and environmental parameters: (i) pulse fluence, (ii) scan line separation, (iii) focal position and (iv) wetting period (due to the measurement of the contact angle). The presented results lead to the following conclusions:

•

•

•

•

Depending on laser fluence, the laser-textured surfaces can develop stable or unstable hydrophobicity; in our case, the stable conditions were achieved if the peak fluence exceeded the threshold fluence of F0 = 12 J cm−2 . In this case, all final contact angles were above 140◦ . If the fluence was below this threshold, the surface first became hydrophobic and after achieving the maximal contact angle, its hydrophobicity decreased by time. The short-term evaluation (e.g., within only 2 months) that is presented by the majority of papers covering this topic, can lead to wrong conclusions, such as stable hydrophilicity for smaller scan line separations or appearance of the wettability gradients due to processing at different focal positions. Here, a long-term examination reveals that such surfaces tend to become hydrophobic after a long-enough period. The presented results indicate that a faster development of hydrophobicity immediately after the laser texturing usually leads to a lower final contact angle and vice versa, if this transition is really slow (as in our case of 10-µm scan line separation), larger contact angles or even superhydrophobic surfaces exhibiting the self-cleaning effect are expected when the transition is over and the stable conditions are achieved. The wetting period due to the measurements of the contact angle influences the hydrophilic-to-hydrophobic transition, but it appears to have no influence on the final wettability (the final contact angle), when stable conditions are achieved.

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/11/11/2240/ s1, Figure S1: Determination of the static contact angle, Figure S2: High-magnification SEM of S68, Figure S3: High-magnification SEM of S18, Figure S4: High-magnification SEM of S32, Figure S5: Self-cleaning effect, Table S1: Parameters of laser texturing with different fluences, Table S2: Parameters of laser texturing with different scan line separations, Table S3: Parameters of laser texturing at different focal positions, Table S4: Parameters of laser texturing for determination of the threshold fluence for laser ablation. Author Contributions: P.G. designed the laser texturing method, performed the laser texturing, wrote the first draft of the manuscript and conducted the research. M.C. and L.H. performed wettability measurements, analyzed the wettability data and contributed in the interpretation of these results. M.H. performed all surface characterization and contributed to the interpretation of the surface analysis results. All authors contributed to the writing of the manuscript and approved the final version of it. Funding: This research was funded by Javna Agencija za Raziskovalno Dejavnost RS (P2-0392, P2-0132, Z2-9215). Acknowledgments: The authors acknowledge the financial support from the Slovenian Research Agency (research core fundings Nos. P2-0392 and P2-0132 and project No. Z2-9215). Conflicts of Interest: The authors declare no competing interests. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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